Carbon fiber reinforced plastic (“CFRP”) materials are increasingly being used in place of metals to form the skin panels and primary structural members of commercial airplanes. CFRP materials are advantageous compared to metals due to the higher strength-to-weight ratios provided by these materials. However, CFRP materials are much less conductive than metallic materials, and as a result require special consideration related to the effects of lightning strikes, static charge buildup, and other electro-magnetic effects (EME). In particular, CFRP materials do not readily provide low-resistance paths to dissipate the electrical potentials that are generated during these events, a condition that can lead to the production of heat and sparks. The poor conductivity of CFRP materials in the primary structure can also result in higher electrical currents in metallic substructure and systems components, requiring additional complexity in those components to prevent the production of heat and sparks. This is particularly important in wing structures that are also used to contain fuel, due to the presence of the flammable fuel and fuel vapors. In these areas prevention of sparking is very important due to the potential for ignition of fuel vapors and the resultant consequences of fire or explosion.
Of particular interest are the interconnections between metallic systems components and the composite structure at locations where there can be large lightning currents transferred across the connections. An example of such an interconnection is the penetration of a composite bulkhead of a fuel tank in a composite wing by a metallic system component such as a fuel line or hydraulic line. The metal system components can be a preferred path for lightning, and the resulting current will be transferred to the CFRP structure at the bulkhead penetration.
At such a location, there is significant potential for generation of heat or sparks during the transfer of current between the metallic system component and the CFRP structure. To prevent this, the generally accepted practice is to provide a stable, reliable current path with electrical resistance that is sufficiently low to prevent heating of the composite material due to the transferred current. Preventing heating of the composite materials prevents the production of high pressure gas and the expulsion of hot particles, commonly observed as a spark.
At these locations, the metallic system components are mechanically attached to the composite structure using metallic bolts or similar fasteners, installed though holes that have been drilled in the composite structure. For practical consideration related to the assembly and maintenance of the aircraft, the holes through which the fasteners are installed are larger in diameter than the fasteners. As a result, the surfaces of the fasteners are not reliably in contact with the inner surfaces of the drilled holes, and do not provide a reliable, low-resistance current path.
Current solutions to prevent heating and sparking involve the use of secondary bonding jumpers and other features to ensure sufficient and reliable conductivity at these locations. Those features add significant weight and complexity to the structure, and their non-integrated nature could result in the features being omitted or otherwise compromised during manufacture or maintenance. Investigation of alternative methods continues to provide additional protection while minimizing additional weight and complexity.
Accordingly, those skilled in the art continue to seek new techniques for providing a good electrical connection to the CFRP to prevent sparking that could result from lightning strikes and static charge accumulation. A robust electrical connection at the wall of a tank is desirable to deter current flow through sensitive components inside the tank and to prevent the occurrence of heating or sparking, which can be particularly hazardous in the fuel-filled structures within an aircraft.